LWN: Comments on "GCC reaches the 4.4.0 release"

GCC reaches the 4.4.0 release

vmole — Tue, 28 Apr 2009 19:39:55 +0000

Those of us who learned to program FORTRAN by punching little holes in paper cards are unlikely to change.

GCC reaches the 4.4.0 release

nix — Sat, 25 Apr 2009 10:09:28 +0000

Less 'missed' more 'forgot that it wasn't in the release notes as its own
page'.

And, yes, I too suspect this will be much much less painful. But it's
still worthy of note.

And now... drumroll please... The great answer!!! It is: YMMV

nix — Sat, 25 Apr 2009 10:04:52 +0000

I know 'measure and see', but the point of an article like this is usually
to collate things like that which others have already done so that we can
get an idea of the magnitude of the benefit *before* we download it. No
article can replace real-world tests, but it could at least report on a
few of them.

You've provided me with more of a clue than the article :) I was wondering
how the 'early optimizers have no understanding of register pressure'
problem had affected this: I know it was part of the design intent of IRA
to try to make this problem less severe...

GCC reaches the 4.4.0 release

dirtyepic — Sat, 25 Apr 2009 04:03:17 +0000

- the caveats. This is the only GCC release that I can remember that breaks enough existing code that a separate page of caveats is needed. The code it breaks is already buggy, but nonetheless...

You must have missed the Porting To page of the GCC 4.3 release notes then. :)

At least in my experience, the C++ header cleanup in 4.3 was responsible for a _lot_ more breakage than anything in 4.4 has thrown our way so far. (though the night is still young)

And now... drumroll please... The great answer!!! It is: YMMV

khim — Fri, 24 Apr 2009 20:53:16 +0000

- graphite and/or the IRA. What effect does this little bugger have on compilation and runtime speed?

The answer is simple: measure and see. IRA usually is able to allocate registers better but graphite often creates additional register pressure. The end result... is mixed. On average gcc 4.4 is better than previous versions of GCC, but some code produces nasty regressions - when loops optimizations introduce so many variables that even IRA can not cope. This is not a big problem on x86-64, but with register-starved architectures (and there are not one but two of then in wide use: IA32 and ARM-Thumb) the only answer is YMMV. Check and see.

I can't see how anyone planning to use GCC 4.4 for serious work wouldn't want to know the answers to these questions, really...

If you plan to use GCC 4.4 for serious work you must grab it and check for yourself. Most programs benefit from GCC 4.4 improvements to one degree or another, but some show regressions. No article can replace real-world tests, sorry...

GCC reaches the 4.4.0 release

nix — Fri, 24 Apr 2009 10:08:59 +0000

I'm afraid this article wasn't as good as it could have been :((

What we got was a compaction of part of the release notes, which you'd
already linked to: anyone interested could have read them anyway.

But what I'm interested in is things not mentioned there:

- the caveats. This is the only GCC release that I can remember that
breaks enough existing code that a separate page of caveats is needed. The
code it breaks is already buggy, but nonetheless...

- graphite and/or the IRA. These are *large* optimizer improvements, with
visible effects on users: graphite drags in some interesting runtime
dependencies on packages that people are very unlikely to have installed
already, and uses Presburger arithmetic (famed for insane time
complexities wherever proof theorists gather in large numbers). What
effect does this little bugger have on compilation and runtime speed? (The
mailing lists probably already contain the answers to this.) The IRA has
not only an unfortunately-chosen acronym that trips up UK and Eire
residents whenever they read it, but also makes major changes to the
register allocator, which is one of the largest things slowing GCC down on
register-starved platforms like i386. What effect does *this* have on
runtime speed?

I can't see how anyone planning to use GCC 4.4 for serious work wouldn't
want to know the answers to these questions, really...

GCC reaches the 4.4.0 release

jordanb — Fri, 24 Apr 2009 04:09:08 +0000

I'm aware that g77 is no longer supported. I was making a wise-ass statement about LWN's archaic reference to 'FORTRAN' rather than 'Fortran', which is the form preferred since the Fortran 90 standard (about 19 years now).

GCC reaches the 4.4.0 release

ssam — Thu, 23 Apr 2009 18:47:21 +0000

g77 has be replaced by gfortran (several releases ago).

gfortran can compile code written in fortran 77.

GCC reaches the 4.4.0 release

jordanb — Thu, 23 Apr 2009 06:17:47 +0000

ITYM "Fortran" the FORmula TRANslation ('77 spec) g77 front end is no longer supported, so I assume those changes are to the gfortran Fortran 90/95/2003 compiler.